write innocent-looking C code implementing malicious behavior. In many ways this is the exact opposite of the Obfuscated C Code Contest: in this contest you must write code that is as readable, clear, innocent and straightforward as possible, and yet it must fail to perform at its apparent function. To be more specific, it should do something subtly evil.

This year's challenge: covert fingerprinting. Write a program that performs some basic image-processing operation, but hides a unique fingerprint in the image it outputs.